The invention relates to a compression-cycle vacuum pump and, more particularly, to a so-called two-shaft vacuum pump which rotates two rotors disposed in a casing, each rotor having at least one protuberance and at least one indentation for cooperatively pumping gas from an inlet port to an outlet port which is associated with one of the two rotors in such a way that the gases being pumped are compressed.
Two-shaft pumps are known from VDI-Zeitschrift, vol. 91, No. 10, of May 10, 1949, and from European Pat. No. 9916. To be able to handle relatively-high pressure differences when using such a pump as a vacuum pump, provision must be made for appropriately-high internal compression. It is then possible, in principle, to produce pressures in the medium-high vacuum range (to 10.sup.-3 millibars) with a pump of this type which works directly against atmospheric pressure. However, this has the drawback that with high suction pressures (at the start of evacuation, for example) internal supercharging occurs which requires high power consumption by the electric motor driving the rotors. To assure reliable operation, therefore, such a pump would have to be provided with a drive motor of sufficient size for this which would be a disadvantage from the point of view of cost and weight of the pump. If internal supercharging had to be coped with only at the start of evacuation, a smaller electric motor could be used since short-time operation with an overload is permissible. However, prolonged overload operation, as in the event of a sizable leak in the system to be exhausted, would result in destruction of the motor, which is why the motor cannot be made smaller if reliable operation is to be assured.
In another type of two-shaft vacuum pump, the Roots pump, there is no internal compression of the gases being moved. This pump is afflicted with the drawback that, with increasing pressure, the compression ratio is reduced. Below 300 millibars, a Roots pump cannot compress air drawn in to atmospheric pressure. This is why a backing pump (such as a sliding-vane or liquid-piston rotary pump) must be used to compress the air to atmospheric pressure. With such a combination of pumps, pressures extending into the high-vacuum range (to 10-.sup.5 millibars) can be produced. However, pump combinations of this type are complicated and expensive.